Ferroelectric liquid crystal (hereinafter referred to as "FLC" as necessary) has a higher order of alignment than the nematic phase, known as "smectic phase," and its state is semi-solid. For this reason, if the alignment of FLC is altered by external pressure, it will not easily return to its original state of alignment. When FLC is provided in a layer with a thickness of a few micrometers or less, the alignment of the molecules is stabilized by the action force of the substrate interface. In such a state, the alignment state of the molecules shows bistability. In a so-called surface-stabilized liquid crystal display element, display is performed using the bistability of the liquid crystal molecules.
When an electric field is applied to FLC, it becomes spontaneously polarized in the smectic phase, and shows biaxial anisotropy of the dielectric constant, and changes the alignment of its molecules due to these two action forces. Consequently, by reversing the spontaneous polarization of the FLC in response to the polarity of an electric field, the FLC can be switched between two stable states at a speed of several microseconds. Further, since switching is performed in the plane of the cell, FLC has characteristics which enable display with a wide viewing angle.
Since, as discussed above, FLC can only be switched between bistable alignment states, only two-value display is possible. For this reason, FLC in a crossed Nicols cell shows, in display, an optical response of light state and dark state. Consequently, in an FLC cell, it is very difficult to perform display at intermediate states other than light state and dark state. Conventional FLC cells which resolved this inconvenience include, for example, the following four methods of performing gray-scale display by changing the average light transmittance.
(1) Frame Division Driving Method
In this driving method, each frame is divided into a plurality of fields of suitable duration, and two-value display is performed in each field (Japanese Unexamined Patent Publication Nos. 6-18854/1994 (Tokukaihei 6-18854) and 5-88646/1993 (Tokukaihei 5-88646)). For example, when each frame is divided in a ratio of 1:2:4, display with eight gradations is possible.
(2) Pixel Division Driving Method
In this method, the pixel electrode of each pixel is divided into a plurality of electrodes of suitable area ratio, and each electrode is driven separately so as to perform two-value display in a sub-pixel corresponding to each electrode (Japanese Unexamined Patent Publication No. 7-5432/1995 (Tokukaihei 7-5432)). For example, when each pixel electrode is divided in a ratio of 1:2:4, display with eight gradations is possible.
(3) Combination of Frame Division and Pixel Division Driving Methods
In this driving method, by combining the two foregoing methods, display with more gradations can be realized (Japanese Unexamined Patent Publication No. 7-152017/1995 (Tokukaihei 7-152017)). For example, when each frame is divided in a ratio of 1:8:64, each pixel electrode is divided in a ratio of 1:2:4, and each sub-pixel is driven so as to perform two-value display, display with 512 gradations is possible.
(4) Threshold Voltage Distribution Driving Method
In this driving method, the proportion of light-state and dark-state domains in each pixel is changed by controlling a group of amplitude-modulated or pulse-width-modulated pulses (Japanese Unexamined Patent Publication Nos. 7-152017/1995 (Tokukaihei 7-152017) and 6-235904/1994 (Tokukaihei 7-235907)). In principle, this method performs two-value driving, but analog-like gray-scale display is realized by providing in each pixel a plurality of domains with liquid crystal with progressively different voltage thresholds.
At present, in gray-scale driving with FLC, display with many gradations is realized by using one of the foregoing driving methods, or by combining several of them together.
The following will explain in outline a representative example of driving using the foregoing driving methods.
In a passive matrix liquid crystal display element, for example that shown in FIG. 15, a plurality of scanning line electrodes 101 . . . and a plurality of signal line electrodes 102 . . . are provided opposite one another and running perpendicularly. A single pixel is formed by each intersection of a scanning line electrode 101 with a signal line electrode 102. In this liquid crystal display element, the state of alignment of each pixel is controlled in accordance with the timing with which strobe pulses (selection pulses) and blanking pulses (erasure pulses) are applied to each scanning line electrode 101, and with a gray-scale signal (or two-value signal) applied to each signal line electrode 102.
To each scanning line electrode 101 are applied, in succession, scanning signals SCAN 1, . . . , SCAN n+1, for example, with the timing shown in FIG. 6. In this example, the aim is to reset the alignment state of the pixel to one of the stable states by means of a blanking pulse P.sub.b with voltage V.sub.b, and then to control the alignment during a selection period by means of a strobe pulse P.sub.s with voltage V.sub.s. The area of each of the pulses P.sub.b and P.sub.s is equivalent, and the two are balanced, from the point of view of direct current (DC balanced), within each scanning period. Further, each blanking pulse P.sub.b forms a pair with a strobe pulse P.sub.s, making up a minimum frame.
The blanking pulse P.sub.b and the strobe pulse P.sub.s may have shapes other than those shown in FIG. 6. For example, provided that erasure and selection functions can be attained, and that the two pulses are balanced from the point of view of direct current, it does not matter if the respective polarities of the blanking pulse P.sub.b and the strobe pulse P.sub.s are reversed in each frame. Again, each of the pulses P.sub.b and P.sub.s may have both positive and negative polarity without any adverse effects.
The scanning signal in FIG. 6 is composed of two sub-frames (scanning periods) per frame: a first sub-frame and a second sub-frame. In this example, the duration from a blanking pulse P.sub.b to the subsequent strobe pulse P.sub.s is set to the same proportion in both the first and second sub-frames. Further, in both the first and second sub-frames, durations T.sub.1 and T.sub.2 from the strobe pulse P.sub.s to the subsequent blanking pulse P.sub.b are set to a ratio of 1:5. Using a scanning signal of this kind, if each sub-frame is capable of, for example, gray-scale display with five gradations, each frame can perform display with 25 gradations.
Further, if each signal line electrode 102 is composed, as shown in FIG. 16, of two electrodes 102a and 102b, provided with a width ratio of 1:2, even more gradations can be realized. For example, if the sub-pixel corresponding to each electrode 102a and 102b is capable of display with five gradations, the pixel composed of these sub-pixels will be capable of display with 13 gradations. If the ratio between T.sub.1 and T.sub.2 shown in FIG. 6 is set to 1:13, display can be performed with 169 gradations per frame.
A scanning voltage V.sub.Y and a signal voltage V.sub.X, having, for example, the wave-forms shown in FIG. 17, are applied to each scanning line electrode 101 and each signal line electrode 102, respectively. Accordingly, a pixel voltage V.sub.Y-X, which is the difference between the scanning voltage V.sub.Y and the signal voltage (gray-scale voltage) V.sub.X, is applied to each pixel.
During a blanking period (erasure period), a group of pulses are formed which have voltage levels of V.sub.b .+-.V.sub.d. Consequently, the liquid crystal is reset to one of its stable states, which it maintains during periods when the blanking pulse P.sub.b is not applied.
During a selection period, a group of pulses are formed which have voltage levels of V.sub.s .+-.V.sub.d. By this means, a voltage with a wave-form having driving characteristics capable of obtaining a predetermined gradation is applied to the FLC in the pixel in question. During non-selection periods, gray-scale signals are applied which select the display of pixels at the intersections of the same signal line electrode 102 with other scanning lines 101, and the group of pulses in these gray-scale signals can, in the pixel in question, maintain the stable state selected during the selection period.
As shown in FIG. 19, each of gray-scale signals DATA0 through DATA4, produced by amplitude modulation, has a different wave-form. When these gray-scale signals DATA0 through DATA4 are used as the signal line voltage V.sub.X, the pixel voltage V.sub.Y-X is as shown in FIG. 18.
In the examples in FIGS. 17 and 18, gray-scale signals are used whose aim is analog gray-scale driving. However, gray-scale display equivalent to that of conventional digital technology can be performed by only using the two wave-forms of these gray-scale signals which realize light state and dark state.
With actual driving methods, in consideration of the liquid crystal's speed of response to driving, and of the duty ratio, the upper limit for frame division is two to three divisions. Again, in consideration of increase of the number of drivers, a suitable number for pixel division is two sub-pixels. Division into three sub-pixels is possible, but this leads to many problems with display characteristics. Because of these limitations of two-value driving, a driving method for FLC gray-scale driving is called for in which the FLC itself shows intermediate shades.
As discussed above, in driving an FLC cell, the liquid crystal can be switched between two bistable states. However, the voltage threshold for shift between the bistable states is not necessarily steep; between the light state and dark state, an intermediate state exists across a voltage range of several volts. Accordingly, with conventional two-value driving, bistable switching was performed using a cell which provided a driving voltage and a mono-domain capable of avoiding this intermediate state.
Since it was comparatively difficult to control this intermediate state, and to obtain stable domain distribution, using conventional driving methods whose object was two-value driving, several methods of obtaining stable domains have been proposed.
For example, Japanese Unexamined Patent Publication No. 6-235904/1994 (Tokukaihei 6-235904) discloses a method of obtaining intermediate shades by distributing domains with different FLC switching thresholds in each pixel using pixel electrodes having a regular slope. Again, Japanese Unexamined Patent Publication No. 7-152017/1995 (Tokukaihei 7-152017) discloses a method of obtaining intermediate shades by distributing domains with different alignment states in each pixel by adding particles to the liquid crystal. Again, in the disclosure of Japanese Unexamined Patent Publication No. 63-201629/1988 (Tokukaisho 63-201629), rubbing processing is performed in two directions on an FLC cell, and a high-frequency pulse is applied to the FLC cell to cause disclination and dislocation, and these result in domain walls, which are controlled to provide multi-domain pixels. Further, Japanese Unexamined Patent Publication No. 9-236830/1997 (Tokukaihei 9-236830; corresponding to U.S. patent application Ser. No. 08/728,200) discusses a method of obtaining intermediate shades by dispersing polymer resin in the FLC, thus forming tiny domains, and changing the distribution of these tiny domains in accordance with a pulse width.
In each of the foregoing methods, a stable intermediate state can be easily obtained by using an amplitude-modulated or pulse-width-modulated driving voltage.
In multiplex driving, a driving signal applied to a given pixel does not always have the same wave-form; patterns to be applied are formed by combining powers of numbers of driving signals having wave-forms which realize intermediate shades. In display, stable intermediate shades must be obtained with each of these combinations. In other words, driving signal wave-forms must be designed giving consideration to the influence of driving signal wave-forms applied during non-selection periods.
Driving signal wave-forms using amplitude modulation or pulse-width modulation have the following problems.
As discussed above, with FLC, spontaneous polarization of the molecules and anisotropy of the dielectric constant act with the applied electric field to cause bistable switching. However, at the time of application of an electric field during a selection period or blanking period, one stable state shifts to the other stable state due chiefly to the action of spontaneous polarization. Thereafter, during non-selection periods, alignment is maintained chiefly by the action of anisotropy of the dielectric constant.
FLC molecules have properties whereby, when the anisotropy of the dielectric constant is positive, they try to shift from their alignment position in the stable state to an alignment even more parallel with the applied electric field, and when the anisotropy of the dielectric constant is negative, they try to shift to an alignment more perpendicular to the applied electric field. This is the reason why liquid crystal switched to a stable state has different apparent memory angles when an electric field is applied thereto and when an electric field is not applied thereto.
During non-selection periods, a gray-scale signal of low voltage, just enough to keep the FLC from switching bistably, is always applied to the FLC. In an example disclosed in Japanese Unexamined Patent Publication No. 5-127625/1993 (Tokukaihei 5-127625), in cases where the value of the gray-scale signal's amplitude voltage differs according to the gray-scale level, in certain display patterns, a low voltage or a high voltage is sometimes continuously applied to a given pixel.
In the foregoing example, as shown in FIG. 20, a voltage of +V.sub.B, -V.sub.B, or 0 is applied to the signal line electrode according to whether the gradation to be displayed is the light, intermediate, or dark state. In this case, since the number of voltages to be applied to a pixel is increased, the action of the anisotropy of the dielectric constant causes the molecules to change their average position, which is the position necessary to maintain a bistable state, and this changes their state of alignment. As a result, the apparent memory angle changes, and this causes change of the quantity of light transmitted. Consequently, even if a certain gradation is selected during a selection period, it becomes impossible to maintain this predetermined gradation in the face of the many combinations of wave-forms of gray-scale signals applied during non-selection periods.
Again, since the FLC molecules always respond to an applied electric field, voltage applied in a period immediately preceding a selection period, for example, will influence switching by voltage application during the selection period. In other words, depending on the way driving voltages are chosen for the signal line electrodes and the scanning line electrodes, switching may become unstable.
With an amplitude-modulated driving voltage, when, as above, a voltage of a certain value is continuously applied, there is a wave-form effect, in which the driving wave-form in a certain period is influenced by the driving wave-form of the immediately preceding period. On the other hand, with a pulse-width-modulated driving signal wave-form like that shown in FIG. 15 (as disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 6-235904/1994), in addition to the influence of this wave-form effect, there are also cases in which combination of wave-forms produces frequency discrepancies in the actual driving signal.
The temperature increase of an FLC cell during driving is proportional to the square of the voltage of the driving signal, and roughly proportional to the frequency of that driving signal. For this reason, with screen display contents which produce the wave-form combination discussed above, the temperature distribution in the screen portion of the FLC cell changes according to the display contents. Since the memory angle and driving characteristics of FLC change with temperature change, in order to stabilize display, it is necessary to hold temperature change in the FLC cell to a minimum.